US20170372903A1 - Method for doping semiconductors - Google Patents

Method for doping semiconductors Download PDF

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US20170372903A1
US20170372903A1 US15/540,618 US201515540618A US2017372903A1 US 20170372903 A1 US20170372903 A1 US 20170372903A1 US 201515540618 A US201515540618 A US 201515540618A US 2017372903 A1 US2017372903 A1 US 2017372903A1
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doping
silicon
diffusion
process according
printing
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Oliver Doll
Ingo Koehler
Sebastian Barth
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Merck Patent GmbH
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Merck Patent GmbH
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/22—Diffusion of impurity materials, e.g. doping materials, electrode materials, into or out of a semiconductor body, or between semiconductor regions; Interactions between two or more impurities; Redistribution of impurities
    • H01L21/2225—Diffusion sources
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/028—Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic Table
    • H01L31/0288—Inorganic materials including, apart from doping material or other impurities, only elements of Group IV of the Periodic Table characterised by the doping material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/186—Particular post-treatment for the devices, e.g. annealing, impurity gettering, short-circuit elimination, recrystallisation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a process and composition for the production of structured, highly efficient solar cells and of photovoltaic elements which have regions of different doping.
  • the invention likewise relates to the solar cells having increased efficiency produced in this way.
  • a silicon wafer (monocrystalline, multicrystalline or quasi-monocrystalline, base doping p or n type) is freed from adherent saw damage by means of etching methods and “simultaneously” textured, generally in the same etching bath. Texturing is in this case taken to mean the creation of a preferentially aligned surface nature as a consequence of the etching step or simply the intentional, but not particularly aligned roughening of the wafer surface.
  • the surface of the wafer now acts as a diffuse reflector and thus reduces the directed reflection, which is dependent on the wavelength and on the angle of incidence, ultimately resulting in an increase in the absorbed proportion of the light incident on the surface and thus an increase in the conversion efficiency of the solar cell.
  • etching solutions for the treatment of the silicon wafers typically consist, in the case of monocrystalline wafers, of dilute potassium hydroxide solution to which isopropyl alcohol has been added as solvent.
  • Other alcohols having a higher vapour pressure or a higher boiling point than isopropyl alcohol may also be added instead if this enables the desired etching result to be achieved.
  • the desired etching result obtained is typically a morphology which is characterised by pyramids having a square base which are randomly arranged, or rather etched out of the original surface.
  • the density, the height and thus the base area of the pyramids can be partly influenced by a suitable choice of the above-mentioned components of the etching solution, the etching temperature and the residence time of the wafers in the etching tank.
  • the texturing of the monocrystalline wafers is typically carried out in the temperature range from 70-less than 90° C., where up to 10 ⁇ m of material per wafer side can be removed by etching.
  • the etching solution can consist of potassium hydroxide solution having a moderate concentration (10-15%).
  • this etching technique is hardly still used in industrial practice. More frequently, an etching solution consisting of nitric acid, hydrofluoric acid and water is used.
  • This etching solution can be modified by various additives, such as, for example, sulfuric acid, phosphoric acid, acetic acid, N-methylpyrrolidone, and also surfactants, enabling, inter alia, wetting properties of the etching solution and also its etching rate to be specifically influenced.
  • These acidic etch mixtures produce a morphology of nested etching trenches on the surface.
  • the etching is typically carried out at temperatures in the range between 4° C. and less than 10° C., and the amount of material removed by etching here is generally 4 ⁇ m to 6 ⁇ m.
  • the silicon wafers are cleaned intensively with water and treated with dilute hydrofluoric acid in order to remove the chemical oxide layer formed as a consequence of the preceding treatment steps and contaminants absorbed and adsorbed therein and also thereon, in preparation for the subsequent high-temperature treatment.
  • the wafers etched and cleaned in the preceding step are treated with vapour consisting of phosphorus oxide at elevated temperatures, typically between 750° C. and less than 1000° C.
  • the wafers are exposed to a controlled atmosphere consisting of dried nitrogen, dried oxygen and phosphoryl chloride in a quartz tube in a tubular furnace.
  • the wafers are introduced into the quartz tube at temperatures between 600 and 700° C.
  • the gas mixture is transported through the quartz tube.
  • the phosphoryl chloride decomposes to give a vapour consisting of phosphorus oxide (for example P 2 O 5 ) and chlorine gas.
  • the phosphorus oxide vapour precipitates, inter alia, on the wafer surfaces (coating).
  • the silicon surface is oxidised at these temperatures with formation of a thin oxide layer.
  • the precipitated phosphorus oxide is embedded in this layer, causing mixed oxide of silicon dioxide and phosphorus oxide to form on the wafer surface.
  • This mixed oxide is known as phosphosilicate glass (PSG).
  • PSG phosphosilicate glass
  • the mixed oxide serves as diffusion source for the silicon wafer, where the phosphorus oxide diffuses in the course of the diffusion in the direction of the interface between PSG and silicon wafer, where it is reduced to phosphorus by reaction with the silicon at the wafer surface (silicothermally).
  • the phosphorus formed in this way has a solubility in silicon which is orders of magnitude higher than in the glass matrix from which it has been formed and thus preferentially dissolves in the silicon owing to the very high segregation coefficient. After dissolution, the phosphorus diffuses in the silicon along the concentration gradient into the volume of the silicon.
  • concentration gradients in the order of 10 5 form between typical surface concentrations of 10 21 atoms/cm 2 and the base doping in the region of 10 16 atoms/cm 2 .
  • the typical diffusion depth is 250 to 500 nm and is dependent on the diffusion temperature selected, for example at about 880° C., and the total exposure duration (heating, coating phase, drive-in phase and cooling) of the wafers in the strongly warmed atmosphere.
  • a PSG layer forms which typically has a layer thickness of 40 to 60 nm. The coating of the wafers with the PSG, during which diffusion into the volume of the silicon also already takes place, is followed by the drive-in phase.
  • composition of the gas mixture here is adapted in such a way that the further supply of phosphoryl chloride is suppressed.
  • the surface of the silicon is oxidised further by the oxygen present in the gas mixture, causing a phosphorus oxide-depleted silicon dioxide layer which likewise comprises phosphorus oxide to be generated between the actual doping source, the highly phosphorus oxide-enriched PSG, and the silicon wafer.
  • the growth of this layer is very much faster in relation to the mass flow of the dopant from the source (PSG), since the oxide growth is accelerated by the high surface doping of the wafer itself (acceleration by one to two orders of magnitude). This enables depletion or separation of the doping source to be achieved in a certain manner, permeation of which with phosphorus oxide diffusing on is influenced by the material flow, which is dependent on the temperature and thus the diffusion coefficient.
  • a typical diffusion duration consisting of coating phase and drive-in phase is, for example, 25 minutes.
  • boron doping of the wafers in the form of n-type base doping a different method is used, which will not be explained separately here.
  • the doping in these cases is carried out, for example, with boron trichloride or boron tribromide.
  • boron trichloride or boron tribromide Depending on the choice of the composition of the gas atmosphere employed for the doping, the formation of a so-called boron skin on the wafers may be observed. This boron skin is dependent on various influencing factors, more precisely to a crucial extent on the doping atmosphere, the temperature, the doping duration, the source concentration and the coupled (or linear-combined) parameters mentioned above.
  • the wafers used cannot contain any regions of preferred diffusion and doping (apart from those which are formed by inhomogeneous gas flows and resultant gas pockets of inhomogeneous composition) if the substrates have not previously been subjected to a corresponding pretreatment (for example structuring thereof with diffusion-inhibiting and/or -suppressing layers and materials).
  • inline doping in which the corresponding pastes and inks are applied by means of suitable methods to the wafer side to be doped.
  • the solvents present in the compositions employed for the doping are removed by temperature and/or vacuum treatment. This leaves the actual dopant behind on the wafer surface.
  • Liquid doping sources which can be employed are, for example, dilute solutions of phosphoric or boric acid, and also sol-gel-based systems or also solutions of polymeric borazil compounds.
  • Corresponding doping pastes are characterised virtually exclusively by the use of additional thickening polymers, and comprise dopants in suitable form.
  • the evaporation of the solvents from the above-mentioned doping media is usually followed by treatment at high temperature, during which undesired and interfering additives, but ones which are necessary for the formulation, are either “burnt” and/or pyrolysed.
  • the removal of solvents and the burning-out may, but do not have to, take place simultaneously.
  • the coated substrates subsequently usually pass through a through-flow furnace at temperatures between 800° C. and 1000° C., where the temperatures may be slightly increased compared with gas-phase diffusion in the tubular furnace in order to shorten the passage time.
  • the gas atmosphere prevailing in the through-flow furnace may differ in accordance with the requirements of the doping and may consist of dry nitrogen, dry air, a mixture of dry oxygen and dry nitrogen and/or, depending on the design of the furnace to be passed through, zones of one or other of the above-mentioned gas atmospheres. Further gas mixtures are conceivable, but currently do not have major importance industrially.
  • a characteristic of inline diffusion is that the coating and drive-in of the dopant can in principle take place decoupled from one another.
  • the wafers present after the doping are coated on both sides with more or less glass on both sides of the surface. More or less in this case refers to modifications which can be applied during the doping process: double-sided diffusion compared with virtually single-sided diffusion promoted by back-to-back arrangement of two wafers in one location of the process boats used. The latter variant enables predominantly single-sided doping, but does not completely suppress diffusion on the back.
  • the current state of the art is removal of the glasses present after the doping from the surfaces by means of etching in dilute hydrofluoric acid.
  • the wafers are on the one hand reloaded in batches into wet-process boats and with the aid of the latter dipped into a solution of dilute hydrofluoric acid, typically 2% to 5%, and left therein until either the surface has been completely freed from the glasses, or the process cycle duration, which represents a sum parameter of the requisite etching duration and the process automation by machine, has expired.
  • the complete removal of the glasses can be established, for example, from the complete dewetting of the silicon wafer surface by the dilute aqueous hydrofluoric acid solution.
  • the complete removal of a PSG is achieved within 210 seconds at room temperature under these process conditions, for example using 2% hydrofluoric acid solution.
  • the etching of corresponding BSGs is slower and requires longer process times and possibly also higher concentrations of the hydrofluoric acid used. After the etching, the wafers are rinsed with water.
  • the etching of the glasses on the wafer surfaces can also be carried out in a horizontally operating process, in which the wafers are introduced in a constant flow into an etcher in which the wafers pass horizontally through the corresponding process tanks (inline machine).
  • the wafers are conveyed on rollers either through the process tanks and the etching solutions present therein, or the etch media are transported onto the wafer surfaces by means of roller application.
  • the typical residence time of the wafers during etching of the PSG is about 90 seconds, and the hydrofluoric acid used is somewhat more highly concentrated than in the case of the batch process in order to compensate for the shorter residence time as a consequence of an increased etching rate.
  • the concentration of the hydrofluoric acid is typically 5%.
  • the tank temperature may optionally additionally be slightly increased compared with room temperature (greater than 25° C. less than 50° C.).
  • Edge insulation is a technical necessity in the process which arises from the system-inherent characteristic of double-sided diffusion, also in the case of intentional single-sided back-to-back diffusion.
  • a large-area parasitic p-n junction is present on the (later) back of the solar cell, which is, for process-engineering reasons, removed partially, but not completely, during the later processing.
  • the front and back of the solar cell will have been short-circuited via a parasitic and residue p-n junction (tunnel contact), which reduces the conversion efficiency of the later solar cell.
  • the wafers are passed on one side over an etching solution consisting of nitric acid and hydrofluoric acid.
  • the etching solution may comprise, for example, sulfuric acid or phosphoric acid as secondary constituents.
  • the etching solution is transported (conveyed) via rollers onto the back of the wafer.
  • About 1 ⁇ m of silicon (including the glass layer present on the surface to be treated) is typically removed by etching in this process at temperatures between 4° C. and 8° C.
  • the glass layer still present on the opposite side of the wafer serves as a mask, which provides a certain protection against overetching onto this side. This glass layer is subsequently removed with the aid of the glass etching already described.
  • the edge insulation can also be carried out with the aid of plasma etching processes.
  • This plasma etching is then generally carried out before the glass etching.
  • a plurality of wafers are stacked one on top of the other, and the outside edges are exposed to the plasma.
  • the plasma is fed with fluorinated gases, for example tetrafluoromethane.
  • fluorinated gases for example tetrafluoromethane.
  • the reactive species occurring on plasma decomposition of these gases etch the edges of the wafer.
  • the plasma etching is then followed by the glass etching.
  • the front surface of the later solar cells is coated with an antireflection coating, which usually consists of amorphous and hydrogen-rich silicon nitride.
  • an antireflection coating which usually consists of amorphous and hydrogen-rich silicon nitride.
  • Alternative antireflection coatings are conceivable. Possible coatings may consist of titanium dioxide, magnesium fluoride, tin dioxide and/or corresponding stacked layers of silicon dioxide and silicon nitride. However, antireflection coatings having a different composition are also technically possible.
  • the coating of the wafer surface with the above-mentioned silicon nitride essentially fulfils two functions: on the one hand the layer generates an electric field owing to the numerous incorporated positive charges, which can keep charge carriers in the silicon away from the surface and can considerably reduce the recombination rate of these charge carriers at the silicon surface (field-effect passivation), on the other hand this layer generates a reflection-reducing property, depending on its optical parameters, such as, for example, refractive index and layer thickness, which contributes to it being possible for more light to be coupled into the later solar cell.
  • the two effects can increase the conversion efficiency of the solar cell.
  • Typical properties of the layers currently used are: a layer thickness of about 80 nm on use of exclusively the above-mentioned silicon nitride, which has a refractive index of about 2.05.
  • the antireflection reduction is most clearly apparent in the light wavelength region of 600 nm.
  • the directed and undirected reflection here exhibits a value of about 1% to 3% of the originally incident light (perpendicular incidence to the surface perpendicular of the silicon wafer).
  • the above-mentioned silicon nitride layers are currently generally deposited on the surface by means of the direct PECVD process.
  • a plasma into which silane and ammonia are introduced is ignited in an argon gas atmosphere.
  • the silane and the ammonia are reacted in the plasma via ionic and free-radical reactions to give silicon nitride and at the same time deposited on the wafer surface.
  • the properties of the layers can be adjusted and controlled, for example, via the individual gas flows of the reactants.
  • the deposition of the above-mentioned silicon nitride layers can also be carried out with hydrogen as carrier gas and/or the reactants alone. Typical deposition temperatures are in the range between 300° C. and 400° C.
  • Alternative deposition methods can be, for example, LPCVD and/or sputtering.
  • the front surface electrode is defined on the wafer surface coated with silicon nitride.
  • silicon nitride In industrial practice, it has become established to produce the electrode with the aid of the screen-printing method using metallic sinter pastes. However, this is only one of many different possibilities for the production of the desired metal contacts.
  • a paste which is highly enriched with silver particles is generally used.
  • the sum of the remaining constituents arises from the rheological assistants necessary for formulation of the paste, such as, for example, solvents, binders and thickeners.
  • the silver paste comprises a special glass-frit mixture, usually oxides and mixed oxides based on silicon dioxide, borosilicate glass and also lead oxide and/or bismuth oxide.
  • the glass frit essentially fulfils two functions: it serves on the one hand as adhesion promoter between the wafer surface and the mass of the silver particles to be sintered, on the other hand it is responsible for penetration of the silicon nitride top layer in order to facilitate direct ohmic contact with the underlying silicon.
  • the penetration of the silicon nitride takes place via an etching process with subsequent diffusion of silver dissolved in the glass-frit matrix into the silicon surface, whereby the ohmic contact formation is achieved.
  • the silver paste is deposited on the wafer surface by means of screen printing and subsequently dried at temperatures of about 200° C. to 300° C. for a few minutes.
  • double-printing processes are also used industrially, which enable a second electrode grid to be printed with accurate registration onto an electrode grid generated during the first printing step.
  • the thickness of the silver metallisation is thus increased, which can have a positive influence on the conductivity in the electrode grid.
  • the solvents present in the paste are expelled from the paste.
  • the printed wafer subsequently passes through a through-flow furnace.
  • An furnace of this type generally has a plurality of heating zones which can be activated and temperature-controlled independently of one another.
  • the wafers are heated to temperatures up to about 950° C. However, the individual wafer is generally only subjected to this peak temperature for a few seconds. During the remainder of the through-flow phase, the wafer has temperatures of 600° C. to 800° C. At these temperatures, organic accompanying substances present in the silver paste, such as, for example, binders, are burnt out, and the etching of the silicon nitride layer is initiated. During the short time interval of prevailing peak temperatures, the contact formation with the silicon takes place. The wafers are subsequently allowed to cool.
  • the front surface electrode grid consists per se of thin fingers (typical number greater than or equal to 68 in the case of an emitter sheet resistance >50 ⁇ /sqr) which have a width of typically 60 ⁇ m to 140 ⁇ m, and also busbars having widths in the range from 1.2 mm to 2.2 mm (depending on their number, typically two to three).
  • the typical height of the printed silver elements is generally between 10 ⁇ m and 25 ⁇ m.
  • the aspect ratio is rarely greater than 0.3, but can be increased significantly through the choice of alternative and/or adapted metallisation processes.
  • An alternative metallisation process which may be mentioned is the dispensing of metal paste.
  • Adapted metallisation processes are based on two successive screen-printing processes, optionally with two metal pastes of different composition (dual print or print-on-print).
  • dual print or print-on-print use can be made of so-called floating busbars, which guarantee dissipation of the current from the fingers collecting the charge carriers, but which do are not in direct ohmic contact with the silicon crystal itself.
  • the back surface busbars are generally likewise applied and defined by means of screen-printing processes.
  • a similar silver paste to that used for the front surface metallisation is used.
  • This paste has a similar composition, but comprises an alloy of silver and aluminium in which the proportion of aluminium typically makes up 2%.
  • this paste comprises a lower glass-frit content.
  • the busbars generally two units, are printed onto the back of the wafer by means of screen printing with a typical width of 4 mm and compacted and sintered as already described in section 5.
  • the back surface electrode is defined after the printing of the busbars.
  • the electrode material consists of aluminium, which is why an aluminium-containing paste is printed onto the remaining free area of the wafer back by means of screen printing with an edge separation less than 1 mm for definition of the electrode.
  • the paste is composed of greater than or equal to 80% of aluminium.
  • the remaining components are those which have already been mentioned under section 5 (such as, for example, solvents, binders, etc.).
  • the aluminium paste is bonded to the wafer during the co-firing by the aluminium particles beginning to melt during the warming and silicon from the wafer dissolving in the molten aluminium.
  • the melt mixture functions as dopant source and releases aluminium to the silicon (solubility limit: 0.016 atom per cent), where the silicon is p + -doped as a consequence of this drive-in.
  • a eutectic mixture of aluminium and silicon which solidifies at 577° C. and has a composition having a mole fraction of 0.12 of Si, deposits, inter alia, on the wafer surface.
  • a highly doped p-type layer which functions as a type of mirror (“electric mirror”) on parts of the free charge carriers in the silicon, forms on the back of the wafer.
  • This potential wall is generally referred to as “back surface field”.
  • edge insulation of the wafer has not already been carried out as described under point 3, this is typically carried out with the aid of laser-beam methods after the co-firing.
  • a laser beam is directed at the front of the solar cell, and the front surface p-n junction is parted with the aid of the energy coupled in by this beam. Cut trenches having a depth of up to 15 ⁇ m are generated here as a consequence of the action of the laser. Silicon is removed from the treated site via an ablation mechanism or ejected from the laser trench.
  • This laser trench typically has a width of 30 ⁇ m to 60 ⁇ m and is about 200 ⁇ m away from the edge of the solar cell.
  • the solar cells are characterised and classified in individual performance categories in accordance with their individual performances.
  • a disadvantage of this (post)doping from such sources is the unavoidable laser damage of the substrate: the laser beam must be converted into heat by absorption of the radiation.
  • the conventional dopant sources consist of mixed oxides of silicon and the dopants to be driven in, i.e. of boron oxide in the case of boron, the optical properties of these mixed oxides are consequently fairly similar to those of silicon oxide.
  • These glasses (mixed oxides) therefore have a very low absorption coefficient for radiation in the relevant wavelength range. For this reason, the silicon located under the optically transparent glasses is used as absorption source. The silicon is in some cases warmed here until it melts, and consequently warms the glass located above it.
  • the silicon is intended to cool again relatively quickly after absorption of the laser radiation as a consequence of the strong dissipation of the heat into the remaining, non-irradiated volume of the silicon and solidify epitactically on the non-molten material.
  • the overall process is in reality accompanied by the formation of laser radiation-induced defects, which may be attributable to incomplete epitactic solidification and thus the formation of crystal defects. This can be attributed, for example, to dislocations and formation of vacancies and flaws as a consequence of the shock-like progress of the process.
  • a further disadvantage of laser beam-supported diffusion is the relative inefficiency if relatively large areas are to be doped quickly, since the laser system scans the surface in a dot-grid process. This disadvantage naturally has less weight in the case of narrow regions to be doped.
  • laser doping requires sequential deposition of the post-treatable glasses.
  • the object of the present invention consists in providing a process and composition for the production of more-efficient solar cells which improve the current yield from the light incident on the solar cells and the charge carriers generated thereby in the solar cell.
  • inexpensive structuring is desirable, enabling the achievement of improved competitiveness compared with doping processes that are currently technologically predominant.
  • the present invention relates to a novel process for the direct doping of a silicon substrate in which
  • a doping paste which is suitable as sol-gel for the formation of oxide layers and comprises at least one doping element selected from the group boron, gallium, silicon, germanium, zinc, tin, phosphorus, titanium, zirconium, yttrium, nickel, cobalt, iron, cerium, niobium, arsenic and lead is printed onto the substrate surface, over the entire surface or selectively, and dried,
  • this step is optionally repeated with a doping paste of the same or different composition
  • doping by diffusion is optionally carried out by temperature treatment at temperatures in the range from 750 to 1100° C.
  • doping of the substrate is carried out by laser irradiation
  • repair of the damage induced in the substrate by the laser irradiation is optionally carried out by a tubular furnace step or in-line diffusion step at elevated temperature, and
  • steps b) to e) can, depending on the desired doping result, be carried out in a different sequence and optionally repeated.
  • the temperature treatment in the diffusion step after laser irradiation is preferably carried out at temperatures in the range from 750 to 1100° C. for the doping, where repair of the damage induced in the substrate by the laser irradiation is carried out at the same time.
  • the present invention also relates to a process as characterised by claims 2 to 9 , which thus represent part of the present description.
  • the present invention also relates to the solar cells and photovoltaic elements produced by these process steps, which, owing to the process described here, have significantly improved properties, such as better light yield and thus improved efficiency, i.e. higher current yield.
  • the increase in charge-carrier generation improves the short-circuit current of the solar cell.
  • the silicon substrate even as indirect semiconductor, is capable of absorbing the predominant proportion of the incident solar radiation.
  • a significant increase in the current yield is only still possible using, for example, solar-cell concepts which concentrate the solar radiation.
  • a further parameter which characterises the performance of the solar cell is the so-called open terminal voltage or simply the maximum voltage that the cell is able to deliver.
  • the level of this voltage is dependent on several factors, inter alia the maximum achievable short-circuit current density, but also the so-called effective charge-carrier lifetime, which is itself a function of the material quality of the silicon, but also a function of the electronic passivation of the surfaces of the semiconductor.
  • the two last-mentioned properties and parameters play an essential role in the design of highly efficient solar-cell architectures and were originally amongst the main factors responsible for the possibility of increasing the performance in novel types of solar cell.
  • Some novel types of solar cell were already mentioned in the introduction. Going back to the concept of the so-called selective or two-stage emitter (cf. FIG. 1 ), the principle can be outlined diagrammatically as follows with reference to its mechanism hiding behind the increase in efficiency, with reference to FIG. 1 :
  • FIG. 1 shows a diagrammatic and simplified representation (not to scale) of the front of a conventional solar cell (back ignored).
  • the figure shows the two-stage emitter, which arises from two doped regions, in the form of different sheet resistances.
  • the different sheet resistances are attributable to different profile depths of the two doping profiles, and are thus generally also associated with different doses of dopants.
  • the metal contacts of the solar cells to be manufactured from such structural elements are always in contact with the more strongly doped regions.
  • the front of the solar cell is provided with the so-called emitter doping.
  • This can be either n-type or p-type, depending on the base material used (the base is then doped in the opposite manner).
  • the emitter in contact with the base, forms the pn junction, which is able to collect and separate the charge carriers forming in the solar cell via an electric field present over the junction.
  • the minority charge carriers here are driven from the base into the emitter, where they then belong to the majorities. These majorities are transported further in the emitter zone and can be transported out of the cell as current via the electrical contacts located on the emitter zone. A corresponding situation applies to the minorities, which are generated in the emitter and can be transported away via the base.
  • the minorities in the base In contrast to the minorities in the base, these have a very short effective carrier lifetime of in the region of up to only a few nanoseconds in the emitter.
  • the recombination rate of the minorities is in simplified terms inversely proportional to the doping concentration of the respective region in the silicon; i.e. the carrier lifetime of the respective minorities in the emitter region of a solar cell, which itself represents a highly doped zone in the silicon, can be very short, i.e. very much shorter than in the base, which is doped to a relatively low extent. For this reason, the emitter regions of the silicon wafer are, if possible, made relatively thin, i.e.
  • the minorities generated in this region which then have a very short lifetime, which is inherent in the system, have sufficient opportunity, or indeed time, to achieve the pn junction and to be collected and separated at the latter and then driven into the base as majorities.
  • the majorities generally have a carrier lifetime which should be regarded as infinite. If it is desired to make this process more efficient, the emitter doping and depth then inevitably have to be reduced in order that more minorities having a longer carrier lifetime can be generated and driven into the base as majorities transporting the current. Conversely, the emitter screens the minorities from the surface.
  • the surfaces of a semiconductor are always very recombination-active. This recombination activity can be reduced very greatly (by up to seven orders of magnitude, measured from the effective surface recombination rate compared with a surface which has, for example, not been passivated) by the creation and deposition of electronic passivation layers.
  • the carrier lifetime of the minorities in these regions becomes so short that their average lifetime only allows an extremely low quasi-static concentration. Since the recombination of charge carriers is based on the bringing together of minorities and majorities, simply too few minorities which are able to recombine with majorities directly at the surface are present in this case.
  • the emitter is still partially responsible for the creation of the electrical contacts to the solar cell, which must be ohmic contacts. They are obtained by driving the contact material, generally silver, into the silicon crystal, where the so-called silicon—silver contact resistance is dependent on the level of doping of the silicon at the surface to be contacted. The higher the doping of the silicon, the lower the contact resistance can be.
  • the metal contacts on the silicon are likewise very strongly recombination-active, for which reason the silicon zone below the metal contacts should have very strong and very deep emitter doping. This doping screens the minorities from the metal contacts, and at the same time a low contact resistance and thus very good ohmic conductivity are achieved.
  • the emitter doping should be very low and relatively flat (i.e. not very deep) in order that sufficient minorities having a sufficient lifetime can be generated by the incident solar radiation and driven into the base as majorities via separation at the pn junction.
  • the present process consists in a simplified production process compared with the two-stage or selective emitter structures described above. More generally, the process describes a simplification of the production of zones doped with different strengths and depths (n and p) starting from the surface of a silicon substrate, where the term “strength” can, but does not necessarily have to, describe the level of the achievable surface concentration. This may be the same in both cases in the case of zones doped in two stages. The different strength of the doping then arises via the different penetration depth of the dopant and the associated different integral doses of the respective dopant.
  • the process described here thus at the same time provides an inexpensive and simplified production of solar cell structures having at least one structural motif which has two-stage doping. Corresponding solar cell structures are as already referred to earlier.
  • the simplified production process is made possible by the use of doping media which can be printed simply and inexpensively.
  • the doping media correspond at least to those disclosed in the patent applications WO 2012/119686 A1 and WO 2014/101989 A1, but may have different compositions and formulations.
  • the doping media have a viscosity of preferably greater than 500 mPa*s, measured at a shear rate of 25 1/s and a temperature of 23° C., and are thus, owing to their viscosity and their other formulation properties, extremely well adapted to the individual requirements of screen printing. They are pseudo-plastic and may furthermore also have thixotropic behaviour.
  • the printable doping media are applied to the entire surface to be doped with the aid of a conventional screen-printing machine. Typical, but non-restrictive print settings are mentioned in the course of the present description.
  • the printed doping media are subsequently dried on in a temperature range between 50° C. and 750° C., preferably between 50° C. and 500° C., particularly preferably between 50° C.
  • the two process cascades described above represent possibilities for the production of two-stage, or so-called selective, dopings.
  • the second embodiment described represents the alternative which is more attractive and to be preferred owing to the smaller number of process steps.
  • the doping action of the printed-on dopant source can be influenced by the choice of the respective process parameters, in particular those of the laser beam treatment or laser beam doping.
  • the doping action can also be crucially influenced and controlled by the composition of the printable dopant source (cf. FIG. 2 ).
  • two-stage dopings can take place not exclusively only through the use of a printable dopant source followed by a further dopant source, but instead they can also be generated through the use of two printable dopant sources.
  • the dose of dopants which is to be introduced into the silicon to be doped can, in particular, be specifically influenced and controlled by the above-mentioned embodiment via the dopant concentrations present in the dopant sources used.
  • FIG. 2 shows a diagrammatic and simplified representation (not to scale) of the doping process according to the invention induced by laser radiation treatment (cf. FIG. 3 ) of printable doping pastes on silicon wafers, where printable doping pastes of different compositions (such as, for example, doping pastes containing different concentrations of dopant) can be employed.
  • printable doping pastes of different compositions such as, for example, doping pastes containing different concentrations of dopant
  • both two-stage dopings and also structured dopings and dopings provided with opposite polarities can be produced very easily in a simple and inexpensive manner on silicon wafers by the process according to the invention using the novel printable doping pastes still to be characterised below, making in total only a single classical high-temperature step (thermally induced diffusion) necessary (cf. FIG. 4 ).
  • the opposite polarities may advantageously both be located on one side of a wafer, or on opposite sides, or finally represent a mixture of the two above-mentioned structural motifs. Furthermore, it is possible for both polarities to have two-stage doping regions, but they do not necessarily have to have both polarities. It is likewise possible to produce structures in which polarity 1 has a two-stage doping, while polarity 2 does not contain a two-stage doping. This means that the process described here can be carried out in a very variable manner. No further limits are set for the structures of the regions provided with opposite dopings, apart from the limits of the respective structure dissolution during the printing process and those which are inherent in the laser beam treatment.
  • FIGS. 3, 4 and 5 depict various embodiments of the process according to the invention:
  • FIG. 3 shows a diagrammatic and simplified representation (not to scale) of the doping process according to the invention induced by laser radiation treatment of printable doping pastes on silicon wafers.
  • the printed and dried-on dopant sources can be sealed with possible top layers in one of the possible process variants.
  • the top layers can be applied to the printed and dried-on dopant sources, inter alia both after the laser beam treatment and also before it. In the present FIG. 5 , the top layer has been supplemented with the printed and dried-on dopant source by thermal diffusion after the laser beam treatment.
  • the present invention thus encompasses an alternative inexpensive process which can be carried out simply for the production of solar cells with more effective charge generation, but also the production of alternative, printable dopant sources which can be produced inexpensively, deposition thereof on the silicon substrate, and selective one-stage and also selective two-stage doping thereof.
  • the selective doping of the silicon substrate can, but does not necessarily have to, be achieved here by means of a combination of initial laser beam treatment of the printed and dried-on dopant source and subsequent thermal diffusion.
  • the laser beam treatment of silicon wafers may be associated with damage to the substrate itself and thus represents an inherent disadvantage of this process inasmuch as this damage, which in some cases extends deep into the silicon, cannot be at least partially repaired by subsequent treatment.
  • the laser beam treatment may be followed by thermal diffusion, which contributes to repair of the radiation-induced damage.
  • the metal contacts (cf. FIG. 1 ) in this type of production of structures doped in two stages are deposited directly on the regions exposed to the laser radiation.
  • the silicon-metal interface is generally characterised by a very high recombination rate (in the order of 2*10 7 cm/s), meaning that possible damage in the strongly doped zone of the region doped in two stages is not significant for the performance of the component as a consequence of the superordinate limiting of the charge-carrier lifetime on the metal contact.
  • This doping can be achieved locally and without further activation of the dopants, as is usually achieved by classical thermal diffusion.
  • the dopant introduced into the silicon can either be driven in deeper or the dopant already dissolved can be driven in deeper and further dopant can subsequently be transferred from the dopant source into the silicon, in the latter case causing an increase in the dose of the dopant dissolved in the silicon.
  • the dopant source printed onto the wafer and dried can have a homogeneous dopant concentration.
  • This dopant source can, for this purpose, be applied to the entire surface of the wafer or printed on selectively.
  • dopant sources of different compositions and different polarities can be printed onto the wafer in any desired sequence. To this end, the sources can, for example, be processed in two successive printing and drying steps.
  • a textured 6′′ CZ wafer with phosphorus base doping, having a resistivity of 2 ohm*cm, is printed with a boron doping paste, as described in the patent applications WO 2012/119686 A1 and WO 2014/101989 A1, using a steel screen (mounting angle)22.5° having a wire diameter 25 ⁇ m and an emulsion thickness of 10 ⁇ m using a doctor-blade speed of 110 mm/s, a doctor-blade pressure of 1 bar and a printing screen separation of 1 mm, where, depending on the other printing parameters, a layer thickness between 100 nm and 400 nm becomes established after complete drying at 600° C. After printing, the printed-on paste is dried for three minutes at 300° C.
  • the wafer is then treated in predefined fields with the aid of an Nd:YAG nanosecond laser having a wavelength of 532 nm and using various laser fluences acting on the dried-on dopant source.
  • the dopings of the various fields on the wafer are subsequently determined with the aid of four-point measurements and electrochemical capacitance-voltage measurements (ECV).
  • ECV electrochemical capacitance-voltage measurements
  • the wafer is subsequently subjected to thermal diffusion in a conventional tubular furnace using an inert-gas atmosphere, N 2 , at 930° C. for 30 minutes.
  • the boron skin formed during the boron diffusion is oxidised after the diffusion, but still during the furnace process, by means of dry oxidation at a constant process temperature and by controlled tilting as a consequence of the introduction of 20% by vol. of O 2 into the process chamber.
  • the sample wafer is freed from glass and oxide layers located on the wafer with the aid of dilute hydrofluoric acid and the doping action is characterised again by means of four-point measurements and electrochemical capacitance-voltage measurements (ECV).
  • ECV electrochemical capacitance-voltage measurements
  • the sheet resistance of the base-doped wafer is 160 ohm/sqr
  • the sheet resistance of a sample field which has been printed exclusively with the paste, but has not been exposed to the laser radiation is 80 ohm/sqr
  • FIG. 6 shows ECV doping profiles as a function of various diffusion conditions: after laser diffusion and after laser diffusion and subsequent thermal diffusion.
  • LD irradiated field 33

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WO2022001294A1 (fr) * 2020-06-30 2022-01-06 常州时创能源股份有限公司 Procédé de préparation d'une batterie se laser
CN114464701A (zh) * 2022-01-17 2022-05-10 常州时创能源股份有限公司 晶硅太阳能电池的扩散方法及其应用

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CN109411341B (zh) * 2018-09-29 2021-07-27 平煤隆基新能源科技有限公司 一种改善se电池扩散方阻均匀性的方法
CN113035976B (zh) * 2021-03-17 2023-01-17 常州时创能源股份有限公司 硼掺杂选择性发射极及制法、硼掺杂选择性发射极电池
CN113471314A (zh) * 2021-05-07 2021-10-01 盐城工学院 一种利用镓掺杂硅纳米浆料制备选择性发射极的方法
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DE102010024308A1 (de) * 2010-06-18 2011-12-22 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Verfahren zur Erzeugung einer selektiven Dotierstruktur in einem Halbleitersubstrat zur Herstellung einer photovoltaischen Solarzelle
DE102012101359A1 (de) * 2011-02-18 2012-08-23 Centrotherm Photovoltaics Ag Verfahren zur Herstellung einer Solarzelle mit einem selektiven Emitter sowie Solarzelle
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CN114464701A (zh) * 2022-01-17 2022-05-10 常州时创能源股份有限公司 晶硅太阳能电池的扩散方法及其应用

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